Mid-Infrared Diagnostics of PAH Processing and Radiation Fields in M33 H II Regions with JWST/MIRI
Abstract
We present JWST/MIRI (Proposal ID 4297) medium-resolution integral field spectroscopy of five H II regions in the nearby spiral galaxy M33, obtained from the Mikulski Archive for Space Telescopes (MAST). The fully calibrated Level-3 spectral cubes provide continuous wavelength coverage from 6.5 - 20.9~μm, enabling analysis of polycyclic aromatic hydrocarbon (PAH) emission, ionic fine-structure lines, and warm molecular hydrogen. The spectra show prominent PAH bands at 7.7, 8.6, and 11.3~μm, multiple H2 rotational transitions, and hydrogen recombination lines. H2 rotational diagrams yield warm molecular gas temperatures of ∼316 - $407$~K. The detected [Ne II], [Ne III], [S III], and [S IV] lines yield ionization indices of −1.41 to +0.98. The PAH ionization fraction shows no clear dependence on radiation hardness, consistent with previous Spitzer results. Unlike previous work, we find no clear correlation between the 7.7/11.2 and 8.6/11.2~μm PAH ratios, while the 7.7/11.2~μm ratio shows a moderate negative correlation with metallicity, possibly due to our small sample (N=5). The PAH-to-VSG ratio decreases with increasing radiation hardness. We derive the ionization parameter γ, far-ultraviolet radiation field, and PAH charge state, finding a predominantly cationic PAH population (∼75%) despite [Ne III]/[Ne II] <1. The lack of a one-to-one correspondence between γ and the 7.7/11.2~μm ratio suggests that PAH ionization is influenced by additional environmental factors. Given the limited sample, these trends are suggestive rather than statistically definitive.
Related
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BibTeX
@misc{porel2026midinfrared,
title = {{Mid-Infrared Diagnostics of PAH Processing and Radiation Fields in M33 H II Regions with JWST/MIRI}},
author = {Puja Porel and Archana Soam and Saikhom Pravash},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22846},
eprint = {2609.22846},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22846},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Mid-Infrared Diagnostics of PAH Processing and Radiation Fields in M33 H II Regions with JWST/MIRI
AU - Puja Porel
AU - Archana Soam
AU - Saikhom Pravash
PY - 2026
DA - 2026/09/19
PB - arXiv
DO - 10.48550/arxiv.2609.22846
UR - https://arxiv.org/abs/2609.22846
AB - We present JWST/MIRI (Proposal ID 4297) medium-resolution integral field spectroscopy of five H II regions in the nearby spiral galaxy M33, obtained from the Mikulski Archive for Space Telescopes (MAST). The fully calibrated Level-3 spectral cubes provide continuous wavelength coverage from 6.5 - 20.9~μm, enabling analysis of polycyclic aromatic hydrocarbon (PAH) emission, ionic fine-structure lines, and warm molecular hydrogen. The spectra show prominent PAH bands at 7.7, 8.6, and 11.3~μm, multiple H_2 rotational transitions, and hydrogen recombination lines. H_2 rotational diagrams yield warm molecular gas temperatures of ∼316 - $407$~K. The detected [Ne II], [Ne III], [S III], and [S IV] lines yield ionization indices of −1.41 to +0.98. The PAH ionization fraction shows no clear dependence on radiation hardness, consistent with previous Spitzer results. Unlike previous work, we find no clear correlation between the 7.7/11.2 and 8.6/11.2~μm PAH ratios, while the 7.7/11.2~μm ratio shows a moderate negative correlation with metallicity, possibly due to our small sample (N=5). The PAH-to-VSG ratio decreases with increasing radiation hardness. We derive the ionization parameter γ, far-ultraviolet radiation field, and PAH charge state, finding a predominantly cationic PAH population (∼75%) despite [Ne III]/[Ne II] <1. The lack of a one-to-one correspondence between γ and the 7.7/11.2~μm ratio suggests that PAH ionization is influenced by additional environmental factors. Given the limited sample, these trends are suggestive rather than statistically definitive.
ER -